This invention relates to a fluid-type torque converter used in the transmission of an automotive vehicle, by way of example. More particularly, the invention relates to a lock-up apparatus in a torque converter for directly mechanically coupling a driving side and a driven side.
In general, a lock-up apparatus of this type is adapted to directly couple a driving-side member and a driven-side member mechanically by connecting a lock-up clutch piston on the driven side and a converter on the driving side via a friction member provided, for example, on the lock-up clutch piston. In this case, the operation of the lock-up piston is controlled by suitably controlling the hydraulic pressure of first and second oil chambers delimited by a clutch plate on either side thereof. When the lock-up apparatus is in the directly coupled state, power from the engine is transmitted with almost no loss due to slip.
In a lock-up apparatus of this kind, shock is produced when the directly coupled state is established (lock-up ON) or when the apparatus is released from the directly coupled state (lock up OFF). Though the shock produced at lock-up ON does not have much effect, the shock produced at lock up OFF is comparatively large and therefore undesirable in terms of drive feeling.
In an attempt to solve this problem, generally the lock-up clutch piston is provided with an orifice for communicating the first and second oil chambers, as disclosed for example in the specification of Japanese Patent Publication (KOKOKU) No. 63-5625. With an orifice of this kind, some of the working fluid supplied to the first oil chamber at lock-up OFF flows into the second oil chamber through the orifice so that the apparatus is gradually released from the directly coupled state. As a result, the shock sustained at lock-up OFF is mitigated.
However, since the orifice is provided in the lock-up clutch piston, the first and second oil chambers are in direct communication via the orifice even in the lock-up ON state. Consequently, the pressure in the second oil chamber leaks through the orifice in the lock-up ON state so that the pressure difference between the second oil chamber and first oil chamber declines. A problem that results is a decline in the transmission torque capacity of the lock-up clutch.